The unsolved killings of a young mother and three teenagers 35 years ago in a small, northern California town haunt the surviving family members of the victims.

But the memory of those murders has also stayed with the community — and especially with the investigators who have re-opened the case of the 1981 attacks on Sue Sharp, her son Johnny, her daughter Tina, and family friend Dana Wingate.

The mystery of what happened in Cabin 28 in Keddie, a faded Sierra Nevada resort community, was revisited Monday in an episode of People Magazine Investigates, PEOPLE’s new true-crime series on Investigation Discovery.

In conversation with PEOPLE Senior Editor Alicia Dennis for the People Magazine Investigates After Show, staff writer Jeff Truesdell talks about how the case stands apart from others he’s reported for the magazine.

Keddie, a once-favored railroad stop for recreational tourists traveling from the San Francisco Bay area, lies just a few miles down the road from the Plumas County seat of Quincy, which has a population of about 2,000.

The bound-and-beaten bodies of Sue Sharp and the two boys were discovered in the Sharp family’s cabin ‘s the morning of April 12, 1981, by Sue’s daughter Sheila, then 14, who had spent the night with a friend next door. Sheila’s sister Tina was missing; her skeletal remains would be found in the woods three years later.

• Watch the full episode of the People Magazine Investigates After Show on the Keddie case on People/Entertainment Weekly Network (PEN). Go to PEOPLE.com/PEN, or download the app for Apple TV, Roku, Amazon Fire TV, Xumo, Chromecast, Xfinity, iOS and Android devices.

Among those who knew the victims were Greg Hagwood, who would grow up to become the current sheriff of Plumas County, and Mike Gamberg, who was then a deputy sheriff and later would be brought back from retirement by Hagwood to reopen the cold case.

“Sheriff Hagwood was a teenager with [Sheila’s] brother Johnny and with Dana Wingate, and the summer before the murders they worked together on a painting crew,” Truesdell says in an exclusive clip from the after show. “Sheriff Hagwood’s mother was a teacher who had Tina in class.”

“Investigator Mike Gamberg was a martial arts instructor at the local college, and in his class were Dana Wingate and Johnny Sharp. The day before the murders, Dana Wingate was at Mike Gamberg’s house,” he says.

“This was personal to them as well.”

Sheila Sharp, 50, now a mother of three and grandmother of two, has gone more than three decades not knowing who committed the murders of her family members. The victims at the cabin were bound together with electrical wire, stabbed with knives and beaten with two different hammers, forensics revealed. Only one hammer was found at the scene.

Mike Gamberg and Sheila SharpJulian Dufort

Those initially interviewed by police included a neighbor of the Sharps, Marty Smartt, who was known to be angry with Sue after she allegedly encouraged his wife to leave him. Police also interviewed Smartt’s ex-con friend “Bo” Boubede, who was sharing the Smartt’s nearby cabin.

Neither man was named a suspect nor arrested at the time, and both soon left the area, never to be questioned again. But in their review of forgotten or overlooked evidence from the voluminous case files — including a letter written by Marty Smart to his then wife and postmarked shortly after the murders that reads, in part, “I’ve paid the price of your love & now that I’ve bought it with four peoples lives, you tell me we are through” — Hagwood and Gamberg believe they may be close to proving the guilt of both men, who have since died.

The investigation has only recently discovered additional evidence, including a blue-handled steel claw hammer that matches a detailed description of one that Smartt said at the time he’d lost. Hagwood and Gamberg say it is being tested for possible DNA or blood residue.

Says Truesdell: “They’re well aware of the fact that the main suspects in their mind are both dead, but they are pursuing confirmation, finally, that those two may have indeed been involved, and in addition, looking for others who may have been accomplices in one way or another, who may have participated in locating Tina’s remains miles and miles away, or may have known about it.”

Because there’s no statute of limitations on murder, “They do feel that they have some potential people of interest that they’re pursuing very aggressively to see if they might yet find someone to answer for this case,” Truesdell says.

People Magazine Investigates continues with new episodes that air Mondays (10 p.m. ET) on Investigation Discovery.

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